tfq.differentiators.LinearCombination

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Differentiate a circuit with respect to its inputs by

Inherits From: Differentiator

linearly combining values obtained by evaluating the op using parameter values perturbed about their forward-pass values.

my_op = tfq.get_expectation_op()
weights = [5, 6, 7]
perturbations = [0, 0.5, 0.25]
linear_differentiator = tfq.differentiators.LinearCombination(
   weights, perturbations
)
# Get an expectation op, with this differentiator attached.
op = linear_differentiator.generate_differentiable_op(
    analytic_op=my_op
)
qubit = cirq.GridQubit(0, 0)
circuit = tfq.convert_to_tensor([
    cirq.Circuit(cirq.X(qubit) ** sympy.Symbol('alpha'))
])
psums = tfq.convert_to_tensor([[cirq.Z(qubit)]])
symbol_values_array = np.array([[0.123]], dtype=np.float32)
# Calculate tfq gradient.
symbol_values_tensor = tf.convert_to_tensor(symbol_values_array)
with tf.GradientTape() as g:
    g.watch(symbol_values_tensor)
    expectations = op(circuit, ['alpha'], symbol_values_tensor, psums
)
# Gradient would be: 5 * f(x+0) + 6 * f(x+0.5) + 7 * f(x+0.25)
grads = g.gradient(expectations, symbol_values_tensor)
# Note: this gradient visn't correct in value, but showcases
# the principle of how gradients can be defined in a very flexible
# fashion.
grads
tf.Tensor([[5.089467]], shape=(1, 1), dtype=float32)

weights Python list of real numbers representing linear combination coeffecients for each perturbed function evaluation.
perturbations Python list of real numbers representing perturbation values.

Methods

differentiate_analytic

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differentiate_sampled

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generate_differentiable_op

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Generate a differentiable op by attaching self to an op.

This function returns a tf.function that passes values through to forward_op during the forward pass and this differentiator (self) to backpropagate through the op during the backward pass. If sampled_op is provided the differentiators differentiate_sampled method will be invoked (which requires sampled_op to be a sample based expectation op with num_samples input tensor). If analytic_op is provided the differentiators differentiate_analytic method will be invoked (which requires analytic_op to be an analytic based expectation op that does NOT have num_samples as an input). If both sampled_op and analytic_op are provided an exception will be raised.

This generate_differentiable_op() can be called only ONCE because of the one differentiator per op policy. You need to call refresh() to reuse this differentiator with another op.

Args
sampled_op A callable op that you want to make differentiable using this differentiator's differentiate_sampled method.
analytic_op A callable op that you want to make differentiable using this differentiators differentiate_analytic method.

Returns
A callable op that who's gradients are now registered to be a call to this differentiators differentiate_* function.

refresh

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Refresh this differentiator in order to use it with other ops.